<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
        }
        /* 里面啥也没有的加属性，
        里面有东西的空格回车符也算，不加 */
        div:empty{
            background: yellow;
        }
    </style>
</head>
<body>
    <div>111</div>
    <div></div>
</body>
</html>